1. Field of the Invention
The present invention relates to the field of refrigeration. More particularly, one embodiment of the present invention relates to a two-stage refrigeration system utilizing an evaporator integrated with an encapsulated thermal energy storage module.
2. Background of Art Related to the Invention
For many decades, domestic refrigerators have included a freezer section and a fresh food section. The fresh food section is maintained at a significantly higher temperature than the freezer section. While the basic laws of thermodynamics provide empirical evidence that it is increasingly more difficult to cool (i.e., remove heat from) an item as its temperature decreases, domestic refrigerators typically have been designed with more consideration focused on cost than thermodynamics. For example, many domestic refrigerators use a one-stage refrigeration system including a single evaporator located in the freezer section. Since the total heat load dissipation is through this single evaporator, this one-stage refrigeration system possesses less than optimal energy efficiency.
Recently, in order to increase system efficiency, some refrigerators have been constructed with two separate refrigeration systems; namely, one refrigeration system is responsible for cooling the freezer section while the other refrigeration system is responsible for cooling the fresh food section. Consequently, this dual refrigeration system includes repetitive condensing units, each featuring a compressor and a condenser. This repetition of equipment increases the cost and size of the refrigerator. Also, these repetitive condensing units produce a greater amount of noise.
Another example involves yacht refrigerators which have been implemented with refrigeration systems having valves to sequentially, but not simultaneously, connect a single, high-capacity condensing unit to multiple evaporators operating at differing temperatures. The refrigeration system may use thermal energy storage (TES) material to provide stable temperatures during the period between evaporator operations.
Preferably, TES material is an aqueous solution such as a salt solution having water and sodium chloride (NaCl). This composition provides high heat storage capacity, emits a large amount of heat isothermally upon changing phase from a liquid to a solid, is non-toxic and can be produced for a low cost. Unfortunately, this TES material is highly corrosive to most metals, tends to expand when frozen which would damage the thin wall of the heat exchanger and tends to freeze first on the heat exchange surfaces which would hamper further heat transfer. This requires the TES material to be separated from the thin-walled metal tubing of the heat exchanger. One technique of separation involves encapsulating TES material into separate expandable capsules as described in U.S. Pat. No. 5,239,839 by the named inventor. However, such encapsulation is costly and difficult to produce.
Additionally, the use of TES material adversely affects the efficiency of conventional defrosting cycles. The reason is that conventional defrost methods, if implemented, would require the entire TES material to melt before actual defrosting could begin.
U.S. Pat. Nos. 4,712,387 and 4,756,164 by the named inventor describe a heat pipe based method for efficiently transferring heat into and out of TES material and a method for thermally de-coupling the TES material from the cooled space to enable simple and efficient defrosting of the evaporator. These methods fail to provide any suggestion of the multi-stage refrigeration system and/or control protocol used to control this refrigeration system.
In contrast to the prior techniques and refrigeration systems, the present application describes a cost-effective evaporation unit and an energy efficient control protocol to maintain steady temperatures for each section of a refrigeration unit. An additional element of this disclosure is the use and design of a simple sensor for determining the frozen fraction of a TES module in order to control on-and-off cycling of the compressor for temperature stabilization.